The answer is that Goldman’s revenue, which is the main driver of bonus pool size, declined 7% from a year ago. Meanwhile employment increased by the same amount, leaving each worker a smaller share of a smaller pie.

This is not a call to weep for starving investment bankers, mind you. Average compensation was still $147,825 in the first quarter, which is more than double the annual median family income in Goldman’s home state of New York.

But if Goldman matches Wall Street’s 2011 revenue estimate of around $38.5 billion and pays out the same proportion of revenue to workers as it did last year — 39% — its bonus pool will shrink for the third straight year, to $423,000.

If that’s not exactly cause for celebration, at least it’s one trend that’s going in the right direction.